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Re: libwww-perl, POE and Unicode

by Juerd (Abbot)
on May 01, 2008 at 03:24 UTC ( #683836=note: print w/ replies, xml ) Need Help??

in reply to libwww-perl, POE and Unicode

eval { $content = $response->decoded_content }; if ($content) { $response->content($content); }

That's bad. HTTP::Message has separate methods for this for a very good reason: An HTTP message can only ever be a sequence of octets. Together with the charset attribute of the Content-Type header, you can decode this sequence of octet, and get a Unicode string from it; the decoded_content method automates this common use. It looks like PoCoCli::HTTP is trying to be helpful and do this so that its users don't have to, but this is a bad idea because:

  • It destroys the coherence between content and Content-Type;charset
  • It destroys the coherence between content and Content-Length
  • It destroys the original octet buffer, which might be very useful (e.g. to re-send octet-verbatim efficiently, or because the charset attribute was incorrect but you know what it should have been)
  • The message is no longer actually HTTP, because HTTP as a TCP protocol is purely octet based
I guess the LWP author got tired of (hearing about) weird bugs caused by sending this kind of corrupt HTTP messages and decided to end this madness. I applaud this decision. Now PoCoCli::HTTP's author has to fix their module. Which, yes, will probably break lots of existing code using PoCoCli::HTTP that depended on this helpfulness. And again we see that innocent users are hurt by someone's ignorance of (or lack of appreciation for) the binary versus text distinction.

Remove these two lines, and use decoded_content instead of content if you need the decoded content.

Juerd # { site => '', do_not_use => 'spamtrap', perl6_server => 'feather' }

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Re^2: libwww-perl, POE and Unicode
by danmcb (Monk) on May 05, 2008 at 09:28 UTC
    the new test that has been added in HTTP::Message is this:
    sub { utf8::downgrade($_[0], 1) or Carp::croak("HTTP::Message content must be bytes") }

    Isn't suddenly deciding to die on something which previous releases didn't even test for a bit, erm, extreme? The thinking, technically speaking, may be completely correct, of course ...

      This is neither just a test, nor extreme. This snippet is exactly what every octet oriented Perl function should do.

      Doubtless, this will "break" some existing code, as it has clearly done in this case. But believe me, clear breakage with a meaningful error message is SO much better than the vague semi-random encoding trouble you could have gotten without it!

      An HTTP message is strictly a stream of octets. It is impossible to have unencoded text, because TCP does not support that. If you would try to use the rejected message over a socket, you would get Perl warnings about "wide characters", a sign that Perl is doing the best it can to force the invalid value into an octet form: it UTF-8 encodes and warns about it. However, Perl first tries to downgrade. If that succeeds, it uses the downgraded string and does not complain. This repairs several bugs caused by sloppy programming.

      The snippet that you cited does the same thing: downgrade and use the downgraded form, but if it cannot be downgraded (that is: if the string contains a character with an ordinal value greater than 255) to byte form, complain. The way in which it complains is different from Perls built in. This code DIES. Your gain: you now know exactly that there is a problem, and where it is. Without this new addition to LWP you would probably not have know that there was a problem, and if you had, you would probably still be looking for the source. And if you're like most Perl programmers who don't yet fully understand the octet/text separation logic, you would probably have mis-identified something else as the source, and "fixed" it by making your code forward incompatible. Much like PoCoCli::HTTP did.

      All I can say is: be glad, very glad, that LWP got this change. The pain it causes is temporarily, and much less than the pain it prevents.

      Juerd # { site => '', do_not_use => 'spamtrap', perl6_server => 'feather' }

        Hi there,

        before posting a bug report, I'd like a comment about why utf8::downgrade() had to be used instead of Encode::encode_utf8().

        The former modifies the input in-place (argh!) and fails if it isn't either Latin-1 or EBCDIC. This means that the behaviour changes depending on the platform, and your odds are gone if your text can't be converted into one of those two encodings (which means a big part of the world).

        The latter would leave its input unchanged, would behave the same whatever the platform (I hope) and would thus be truly "octet oriented".

        Do you think a bug report/patch would be well-placed in this case, or am I overlooking something?


        perl -ple'$_=reverse' <<<ti.xittelop@oivalf

        Io ho capito... ma tu che hai detto?

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